A Dual-Gauge Petrol Electric Locomotive using Brushless Motors in 71/4" and 5" Gauge
June 2018 - We have a loco, let's roll!
Dual gauge is by means of having built two pairs of bogies. So far this year the loco has run in excess of 100km on the 5" bogie sets.
The power source is a Honda GX120 petrol engine. This belt-drives a 1.8kW three-phase brushless motor used as a generator - this forms the easily removable power unit.
The brushless motor/generator with separate rectifier was chosen for the high circa 90% efficiency - much more efficient than typical vehicle alternators. However, unlike vehicle alternators, there is no voltage regulator and output voltage varies from a low of around 20v at tickover to a high of around 55v at high revs. This is not any problem here as the motor drive electronics use the new STM3 Brushless motor controllers, designed to operate over a very wide supply voltage range.
The electronics controls the Honda engine using a model control servo, and the train typically purrs around the track with the engine running not much faster than tickover. In practice, the higher engine revs needed to generate the highest voltages are rarely needed, only when pulling heavy loads up gradients at speed.
There is no battery in this design - this was one of the design criteria. The loco is started-up using the pull-cord starter on the Honda. Once this starts, power is available for the control and other systems. So far this has proved very reliable. The engine starts on the first pull and has run for periods of several hours at a time.
The longest run so far was on 28th May at Ashton Court, Bristol. The engine ran for more than the whole 5 hours of 'public running', covered a distance of 16 miles, consuming a little over one litre of petrol.
Total distance travelled to 16th June = 62 miles, all using the 5" gauge bogie sets.
Here are a couple of vids of Bob driving at Ashton Court, Bristol, 28th May 2018
More to follow soon, all below here is old stuff cobbled together during the design and build, a bit of the history.
Bogies completed October 2016
Frame length 1440mm. A bit short, but then it has to fit in the car with all the rest.
But what of batteries?
There are plenty of locomotive designs that use some generator with batteries - same idea as hybrid cars. This is justified where batteries alone would unduly limit range, or where the generator has insufficient power for all loads and conditions.
The petrol generator shown here is easily removable - the whole point of modular design is to allow for quick and easy assembly / disassembly for one-man handling and ease of transport.
A pair of 12V 50AH gel batteries (as used in mobility scooters) could be dropped into the well in place of the petrol gen, and there we have a battery-electric locomotive !
There's plenty of space to fit up to another six such batteries, 3 in front of and 3 behind the centre well, however the wheelbarrow full of cash to pay for these is absent.
Another option - batteries could be fitted in front of and behind the petrol generator.
Flexibility is part of the design philosophy, multiple alternative power sources, wider range of possible uses.
Electric motors are also usable as brakes. Using a system of 'Regenerative Braking', the motors can also behave as generators, but where is this regenerated power to go?
In a battery-electric design, some of this power can be put back into the batteries - very good, but this is not effective with fully charged, or near fully charged, batteries. Without anywhere else to 'dump' any unusable regenerated power, the system voltage can easily rise beyond safe limits. Any robust electric locomotive design will therefore, include some over-voltage sensing, and voltage limiting, power dump.
Design calculations indicate the petrol gen, electronics and brushless motors alone will give more than enough poke for useful duty hauling passengers around a club track, and a full tank of petrol should last a long afternoon or thereabouts.
Calculations also indicate the torque deliverable to the wheels is well above that likely to induce wheel-slip. Should this prove a problem, batteries may be added - as ballast !
A spreadsheet of design calculations can be seen here As you can see, the loco should be able to accelerate a load of 2 tons up a 2% gradient. Please download and play with loads and gradients etc.
Video shows one bricked-up bogie on the test bench.
The bench power supply is only capable of delivering 5 Amp at 31 Volts. In regular use, the bogie may be expected to draw up to about 35 Amp from 48 Volts. Note when the hand appears and slows the axle, this is because the bench supply collapses under the load !
The slider control seen to the right side of the touch-screen display will be the normal drivers control. Moving a finger over the slider towards the top and the loco will move. Taking a finger off the slider causes the control to drift back to the 'neutral' position and the loco will glide gracefully to a halt. This meets the 'Dead Mans' function required of electric locomotives on club tracks. For purposes of this test, the buttons 'hidden' behind the voltmeter and ammeter are used to increase / decrease speed.
Below, a very simple power dump for use with regenerative braking.
When the applied voltage is below the zener diode voltage, no voltage is developed across R1, no voltage is applied to the mosfet gates so nothing conducts. As applied voltage rises, the zener passes current developing a voltage across R1 which is applied to the mosfet gates. Once this voltage exceeds the gate threshold voltage, current starts flowing through the mosfets. Resistors R2, R3 etc tend to equalise current through each, and also tailor a not too vicious response to rising voltage.
Any number of mosfets with equalising resistors may be connected in parallel, for this loco design there are 20, each capable of dumping around 50 Watt. (I had considered designs using high power wire-wound resistors, mosfets are much less expensive !).